1. Field of the Invention
This invention relates to a process for the production of hydrogen. More specifically, this invention relates to a catalytic process for the production of hydrogen at low temperatures for use in methanol or proton exchange membrane fuel cells.
2. Description of the Related Art
Fuel cells combine hydrogen and oxygen without combustion to form water and to produce direct current electric power. The process can be described as electrolysis in reverse. Fuel cells have been pursued as a source of power for transportation because of their high energy efficiency, their potential for fuel flexibility, and their extremely low emissions. Fuel cells have potential for stationary and vehicular power applications; however, the commercial viability of fuel cells for power generation in stationary and transportation applications depends upon solving a number of manufacturing, cost, and durability problems.
The most promising fuel cells for widespread transportation use are Proton Exchange Membrane (PEM) fuel cells. PEM fuel cells operate at low temperatures, produce fast transient response, and have relatively high energy density compared to other fuel cell technologies. Any fuel cell design must: (a) allow for supply of the reactants (typically hydrogen and oxygen); (b) allow for mass transport of product (water) and inert gases (nitrogen and carbon dioxide from air), and (c) provide electrodes to support catalyst, collect electrical charge, and dissipate heat.
Proton exchange membranes (PEM) fuel cells that typically utilize Pt on carbon support (Pt/C) as anode electrocatalyst operate at a lower temperature of 80xc2x0 C. hold commercial promise. For methanol fuel cells, H2 feed can be produced via one of the following reactions:
CH3OH+H2Oxe2x86x923H2+CO2 xcex94H=+49.4 kJ.molxe2x88x921xe2x80x83xe2x80x83(1)
CH3OH+xc2xdO2xe2x86x922H2+CO2 xcex94H=xe2x88x92192.2 kJ.molxe2x88x921xe2x80x83xe2x80x83(2)
Steam reforming of methanol in Reaction 1 is carried out at temperatures greater than 280xc2x0 C. over supported Cu/Zn catalysts as described by Velu, Suzuki and Osaki in Chem. Communications, No. 23, 2341-2342 (1999). Partial oxidation of methanol in Reaction 2 is also feasible and the reaction is exothermic. See Cubeiro and Fierro in Journal of Catalysts, 179, 150-162 (1998). However, a shortcoming of the above process is that the hydrogen feed produced in this manner has a high content of carbon monoxide (CO). It is known that Pt is readily poisoned by CO. Therefore, a major challenge to the commercializing of the PEM fuel cell technology is to produce H2 that is essentially free of CO. Several catalysts of the type Ptxe2x80x94Ru/C or Ptxe2x80x94Mo/C, have been formulated to increase CO tolerance of the Pt catalyst as discussed in a review article by Mukerjee, et al., Electrochemical and Solid-State Letters. 2(1) 12-15 (1999). But even at a CO content of 100 ppm in the H2 feed, severe catalyst poisoning is observed.
H2 produced via Reaction 1 or 2 contains more than 100 ppm CO. Currently, a catalytic water-gas-shift (WGS) step as illustrated by Reaction 3 is added to remove CO to acceptable levels ( less than 20 ppm) prior to feeding H2 to the fuel cell.
xe2x80x83CO(g)+H2O(g) less than = greater than H2(g)+CO2(g) xcex94H=xe2x88x9239.4 kJ.molxe2x88x921xe2x80x83xe2x80x83(3)
Reaction 3 is typically catalyzed by promoted iron oxides at temperatures greater than 300xc2x0 C. as discussed by C. L. Thomas, in xe2x80x9cCatalytic Processes and Proven Catalystsxe2x80x9d, Academic Press, New York, 1970. As a result, such high temperature pretreatment unnecessarily adds cost to the process. Moreover, in the gas phase, Reaction 3 is in an equilibrium that invariably leaves some CO in the product H2 stream.
Accordingly, there is still a need in the art of PEM fuel cells to utilize hydrogen that is essentially free of carbon monoxide. Additionally, there is also a need to provide the hydrogen gas in a process that is conducted at low temperature by using inexpensive and simple methods.
It is, therefore, an object of the present invention to provide an improved process for the production of hydrogen gas.
It is a further object of the invention to provide a catalytic process for the production of hydrogen gas which contains reduced carbon monoxide content.
The present invention, which addresses the needs of the prior art, provides a process for the catalytic production of a hydrogen feed by exposing a hydrogen feed to a catalyst which promotes a water-gas-shift reaction in a liquid phase. The hydrogen feed can be provided by any process known in the art of making hydrogen gas. It is preferably provided by steam reforming or oxidation of methanol or by any other process that can produce a hydrogen feed for use in proton exchange membrane fuel cells. The step of exposing the hydrogen feed takes place preferably from about 80xc2x0 C. to about 150xc2x0 C. Formate is formed when the water-gas-shift reaction is base catalyzed.
The catalyst used in the process of the present invention can be selected from homogenous transition metal complexes. The transition metal of the complex is preferably a metal selected from Group VIII A of the periodic table, including, for example, Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt and Cu. The transition metal can be coupled to at least one N donor ligand such as 2,2xe2x80x2-dipyridyl (BIPY), sodium salt of ethylenediamine tetraacetic acid, ethylenediamine, 1,10-phenanthroline, 4,4xe2x80x2-dipyridyl, 1,4,8,11-tetraazacyclotetradecane (CYCLAM), N,N-Bis(2-hydroxybenzyl)ethylenediamine H4 (SALEN), or mixtures thereof. The catalytic process of the invention is carried out preferably in a highly basic liquid phase such as provided by water, methanol, glyme, polyglycol, other alcohols from C2 to C10 or ethers from C2 to C10 and mixtures thereof. The liquid phase is made basic by adding bases in an amount sufficient to promote formate formation. The pH of the liquid phase is preferably greater than 8.
As a result of the process of the present invention, a new integrated system that operates at low temperatures is provided. The system consists of two steps: 1) catalyzed methanol decomposition at a temperature of less than 150xc2x0 C. to produce 1 mol CO and 2 mol H2 followed by, 2) fast and complete CO conversion to CO2 with concomitant production of 1 mol of H2 via the present invention. The present integrated system thus produces 3 mol H2/mol methanol at low temperature of less than 150xc2x0 C. compared to schemes for methanol fuel cell systems that are under development.
Other improvements which the present invention provides over the prior art will be identified as a result of the following description which set forth the preferred embodiments of the present invention. The description is not in any way intended to limit the scope of the present invention, but rather only to provide the working example of the present preferred embodiments. The scope of the present invention will be pointed out in the appended claims.
The present invention is a process for the catalytic production of hydrogen feed at low temperatures for use in proton exchange membrane fuel cells. More specifically, the gaseous feed formed by the process of the present invention is hydrogen rich and contains very low levels of carbon monoxide.
In the process of the present invention a hydrogen feed can be formed by any process known in the art. A hydrogen feed is preferably formed by steam reforming or oxidation of methanol, methane or biomass. Hydrogen feed can also be obtained from gasification of coal and other carbonaceous materials including, without limitations, wastes of organic materials, plastics, farm, wood chips and other industrial wastes. Once formed, the hydrogen feed is exposed to a catalytic liquid phase homogeneous systems to achieve a water-gas-shift reaction for CO removal to levels less than 50 ppm. In the reaction known as water-gas-shift, water is reacted with carbon monoxide to yield hydrogen and carbon dioxide. This reaction is shown below:
CO(g)+H2O(1) less than = greater than H2(g)+CO2(g) xcex94H=+2.8 kJ.molxe2x88x921xe2x80x83xe2x80x83(3A)
This reaction operates at a low temperature of less than 150xc2x0 C. CO is dissolved in the liquid phase and reacts with water on a homogenous catalytic system to produce H2 and CO2.
For application to PEM fuel cells, two requirements must be met. These are: 1) the reaction preferably operates at a lower temperature of from about 80xc2x0 C. to about 150xc2x0 C.; and 2) CO removal to less than 50 ppm is achieved with fast reaction rates. In studies reported in literature, the mechanism of homogeneously catalyzed WGS reaction has been established. For example, in base-catalyzed WGS reactions, formate ion is invoked as an intermediate as shown in Reactions 4 and 5 below:
xe2x80x83CO+xe2x88x92OHxe2x86x92HCO2xe2x88x92xe2x80x83xe2x80x83(4)
HCO2xe2x88x92+H2Oxe2x86x92H2+CO2+xe2x88x92OHxe2x80x83xe2x80x83(5)
The sum of Reactions 4 and 5 is the WGS reaction (3) above. Thus, catalyzed formate decomposition is also a measure of WGS activity of a catalyst.
The advantage of the present invention is provided by the thermodynamic advantage of Reaction (3A) as opposed to Reaction (3) above. In the prior art the WGS reactions are in the gas phase. As a result of a negative enthalpy, Reaction (3) tends to go backwards to produce large amounts of CO. In the present invention, the mechanism illustrated in Reaction (3A) indicates that the reaction goes only in forward direction because in the liquid phase the homogenous catalyst reacts with CO and then picks up water to form CO2. That is why by using a catalytic liquid phase homogenous system almost 100% of CO is converted to CO2.
Commercially available from Aldrich Corp. and several other vendors, several metals (heterogeneous) and metal complexes (homogeneous) have been employed as catalysts useful in the present invention. Under basic conditions, at pH greater than 8, formate formation is facilitated as shown in Equation 6 below:
AMxe2x80x94OH+COxe2x86x92AMxe2x80x94HCO2xe2x80x83xe2x80x83(6)
(AM=Li, Na, K, Cs)
Thus, Equation 7 is a part of the WGS catalytic cycle:
AMxe2x80x94HCO2+H2Oxe2x86x92AMxe2x80x94OH+H2+CO2xe2x80x83xe2x80x83(7)
Useful sources of formate include formate salts of lithium, sodium, potassium and cesium, all readily available commercially or these materials can be conveniently synthesized in batches in the laboratory according to procedures well known in the art. Useful bases for inclusion in the liquid phase include, without limitation, hydroxides, alkoxides, bicarbonates of lithium, sodium, potassium and cesium. Alkyl amines wherein the alkyl group is from C1-C4 are also useful bases for the purposes of the present invention. A preferred base is potassium hydroxide.
In the present invention, commercially available transition metal complexes, based on Ru, Ni, Rh, Pt, Co, Fe, Pd, Os, Ir, Cu metals in methanol/H2O solvent mixture are employed. Useful transition metal complexes for this invention are easily commercially available and include without limitation RuCl3.xH2O, Ru3(CO)12, NiCl2.6H2O, RhCl3.3H2O, CoCl2,K2PtCl4, FeCl2, Ru(CO)5, Ni(CO)4, Rh6(CO)16, Co2(CO)8, [Pt(CO)(Cl2)]2 and mixtures thereof. For RuCl3.x H2O, x is an integer between 0 to 3.
A preferred catalyst is formed by dissolving RuCl3xH2O with a water-soluble ligand such as 2,2xe2x80x2-dipyridyl (BIPY) as manufactured by Aldrich, a commercial vendor. An organic solvent may be added such as methanol, ethanol and the like. Listed in Table 1 below are experiments that show the activity pattern of various catalysts useful in formate decomposition.
The experiments summarized in Table 1 were obtained by catalyzing 50 mmol of KHCO2 in 130 ml of a solvent mixture of 5% H2O/10% methanol/85% triglyme, all placed in a 0.5L AE Zipperclave batch reactor at a temperature of 120xc2x0 C. and a pressure of 1.4 MPa.
From Table 1 above, it is apparent that the preferred catalyst system contained Ru and N-donor ligands. N-donor ligands useful in the catalyst system of the present invention include but not limited to are 2,2xe2x80x2-dipyridyl (BIPY), sodium salt of ethylenediamine tetraacetic acid, ethylenediamine, 1,10-phenanthroline, 4,4xe2x80x2-dipyridyl, 1,4,8,11-tetraazacyclotetradecane (CYCLAM), N,N-Bis(2-hydroxybenzyl)ethylenediamine H4 (SALEN).
The solvent system typically employed for the homogenous catalysts useful in the present invention is an organic and/or aqueous solvent such as methanol, ethanol, other higher alcohols, glymes, polyglycol, water and mixtures thereof. H2 is produced with extremely fast reaction rates. Turnover numbers as high as 8 mol H2/mol Ru/min have been obtained. When the catalyst system also contains N-donor ligands turnover numbers are enhanced and can vary from about 0.1 to about 12 mol H2/mol metal/min. Applying the process of the present invention to a gaseous stream of CO, H2O and H2 and CH3OH results in removal of CO to very low levels. For example, levels of carbon monoxide well below 50 ppm, and preferably less than 20 ppm can be achieved.
In the examples that follow, essentially complete formate decomposition as well as CO to CO2 oxidation with H2O is demonstrated. Such a system allows removal of CO to well below the 50 ppm level from a gas stream containing CO, H2O, H2, CH3OH.